Universal Terrestrial Radio Access Network (UTRAN) is a conceptual term identifying a part of a radio communication network, wherein a plurality of mobile terminals communicate with each other or with a terminal in a PSTN or in a packet network (e.g. Internet) through one or more base stations. In particular, UTRAN identifies part of the network, which consists of radio network controllers (RNCs) and Node Bs between interconnection (Iu) and the radio interface (Uu). The interconnection Iu is an interface between an RNC and a core network, and the radio interface Uu is between UTRAN and the user equipment. This forms the basic architecture for the third generation mobile phone system UMTS (Universal Mobile Telecommunication System). The architecture of UMTS will include UTRA for radio access.
One of the modes of UTRAN for the user equipment (UE) is the FDD (Frequency-Division Duplex) mode, as distinguished from the time-division duplex (TDD) mode. UE radio transmission and reception (FDD) is described in the Technical Specification (TS) 25.101 v 3.1.0 (1999-12) of the Third Generation Partnership Project (3GPP) and documents referenced therein.
It is known that in the third-generation (3G) mobile telecommunications system, wideband code division multiple access (WCDMA) has emerged as the mainstream air interface solution. In a WCDMA system, information bits are spread over a wide bandwidth by multiplying data with quasi-random bits (chips) derived from CDMA spreading codes, i.e. the channelization codes. In Universal Terrestrial Radio Access (UTRA), the data generated at higher layers is carried over the air on transport channels, which are mapped onto the physical code channels. Different types of transport channels exist, namely common channels, dedicated channels and shared channels. In the cell_DCH state, the dedicated channel (DCH) is used to carry all the signaling messages and all the information from higher protocol layers, including data for the service as well as control information. There are currently six different common transport channels: Broadcast Channel (BCH), Forward Access Channel (FACH), Paging Channel (PCH), Random Access Channel (RACH), Uplink Common Packet Channel (CPCH) and Downlink Shared Channel (DSCH). The RACH channel is intended for signaling messages from the terminal, such as a request to set up a connection. The CPCH is an extension to the RACH that is intended to carry packet data in the uplink direction. In the cell_FACH state, the counterpart of the uplink RACH and CPCH is FACH in the downlink. The FACH is used to carry control information and signaling messages to the mobile terminals known to locate in a given cell or in a given paging area. The DSCH is used to carry user data. The information carried on a DSCH is typically dedicated to a user, but the DSCH could be used to carry data that is shared by several users e.g. multicasting. The DSCH is always associated with an uplink and downlink DCH, noted as associated DCH (aDCH). The aDCH carries the physical control information between the terminal and the base station.
It is well known that the Protocol Data Units (PDUs) from higher layers are carried over the air interface by the transport channels. Different transport channels are multiplexed onto the physical codes. Physical channels and transport channels are specified in 3GPP TS 21.211 V3.4.0 (2000-9). In particular, the DSCH transport channel is mapped onto the Physical Downlink Shared Channel (pDSCH); the CPCH is mapped to the Physical Common Packet Channel (pCPCH); and the DCH is mapped to the dedicated physical channel DPCH, which is a code multiplex of the Dedicated Physical Data Channel (DPDCH) and the Dedicated Physical Control Channel (DPCCH) in the uplink, and a time multiplex of the Dedicated Physical Data Channel (DPDCH) and Dedicated Physical Control Channel (DPCCH) in the downlink.
In the physical transport, scrambling codes are used in addition to the channelization codes, so that signals from different sources can be separated from each other without changing the signal bandwidth and to generate lower cross correlation products. Transmission from a single source can be separated by the orthogonal channelization codes (spreading codes). The codes are selected from a code tree (Orthogonal Variable Spreading Factor codes) to satisfy the orthogonality condition. Channelization codes provide the spectrum spreading sequences. Orthogonal codes are selected for different transport bitstreams so that despreading a received code can give high signal-energy to noise-energy ratios. The scrambling codes separate signals from different terminals in the uplink. The scrambling codes separate signals from different cells or cell sectors in the downlink.
The DSCH transport channel is capable of multiplexing a large number of bearers either in time division manner, in code division manner or both. The time division is done so that the transmission is divided to scheduling periods, and for each period a scheduler decides which bearer, or which radio link control (RLC) buffer, to transmit. The code division is carried out so that a DSCH code sub-tree is further divided to multiple pDSCH code sub-trees. Allocation of each pDSCH is decided by the corresponding scheduler. An example of pDSCH code sub-tree is shown in FIG. 1. When a code-tree node is reserved for the DSCH, it may be entirely applied to transmit on one physical code for one terminal during a scheduling period. Alternatively, it may be code-divided to transmit for several terminals each on a separate physical code pDSCH during the same scheduling period, thus several physical codes are allocated below the DSCH code-tree node at the same time. For example, the code-tree node 110 at SF=4 is reserved for the DSCH to transmit on one physical code for one terminal. However, all the nodes 111-116 in the branches beneath the code-tree node 110 in this code sub-tree can be used to allocate different physical codes so that several terminals can receive different physical codes during one scheduling period of time. The physical codes allocated in the nodes 111 and 112 are orthogonal as the nodes 113-116 are orthogonal codes. As mentioned earlier, the DSCH is always associated with an aDCH. This means that every terminal, which uses the DSCH and is subject to DSCH scheduling always sets up the aDCH in the uplink and in the downlink. The dedicated physical control (DPCCH) is always on the DCH. However, the data PDUs (protocol data units) can be on the DCH or on the DSCH depending on the scheduling and the MAC (medium access control) switching between the DCH and DSCH. The DCH nodes are allocated on the code-tree in the part of the tree that is outside of the DSCH reservation. As shown in FIG. 1, the aDCH can be allocated on the many nodes in many branches outside of the branch under code 110. For example, nodes 122, 155, 153 and 154 can be used to allocate the physical codes for the aDCHs of different terminals. However, nodes 120, 151 and 152 are blocked because these nodes would not preserve orthogonality to the already allocated (122, 155, 153, 154) codes.
The maximum bitrate that will be available on the DSCH is high, because a low spreading factor node in the code tree can be reserved for it. This is because reserving a node for DSCH allows multiple terminals to use the same code. If this same node would be reserved for a DCH, all other terminals would be blocked. Thus DSCH uses the code tree more efficiently than a DCH. Hence, the DSCH is capable of providing high throughput and lower packet delays for a large number of bursty bearers having simultaneous sessions.
For web-type traffic and TCP sessions, packet generation is typically very bursty with short, high peak bitrate active periods and long in-activity periods. For web sources, there are short bursts of high volume data or high bitrate data, and long periods of no activity, e.g. during the webpage “reading” time. For traffic like this, the dedicated transport channel resources are not used effectively. If this kind of traffic is carried on a DCH, it requires setting up and releasing codes frequently. The penalty of doing this is the delay caused by DCH setup signaling and synchronization. If, on the contrary, the code release time is long, there will be no delays, but the code allocation reserves code capacity from the downlink code-tree. In the downlink code-tree, this can cause fairly low spreading factors to be kept allocated, if the expected bitrate is high. This implies increased blocking probability for the other co-existing bearers.
The DSCH gives a clear benefit as to the code resource allocation, as it occupies only a single node in the code-tree per pDSCH. Allocation of this code is efficient, as it will be used for the bearer having heaviest need for data transport at a given scheduling period of time. Between sequential allocation periods, different bearers can be switched and transmitted on the DSCH without any delays. The physical layer TFCI (Transport Format Combination Indicator) signaling is present on the aDCH and it indicates that the transport of the PDUs is actually on the DSCH. TFCI is transmitted in the dedicated physical control channel to inform the receiver which transport channels are active for the current frame and the coding and bitrate mapping in each transmission time interval. The current solution, for switching bearers to the DSCH requires that for every UE, the aDCH is set up, as shown in FIG. 2. This can carry DPCCH for pilot symbols, power control commands and for the TFCI information. It may also carry the DPDCH for the periods of time when the DSCH is not allocated for this particular UE. As shown in FIG. 2, the UE in the cell_FACH state has to receive the aDCH setup message on the FACH, and the UE (and the corresponding network entity) can change from the cell_FACH state to the cell_DCH state during the setup procedure. The settings for the associated DCH may be too demanding for some traffic sources from the cell code-tree resource point of view. Similar problems exist also when trying to allocate any DCH for a high-bitrate bearer. If the release timer for it is long, it unnecessarily consumes code-tree space, and if the spreading factor is low, it badly blocks other bearers. On the other hand, if the release timer is short, the code will be released frequently and every activity period will require setting up the DCH again before data transmission can start. Again, this causes the DCH setup delay. The same applies for the DSCH, as setting up the aDCHs is necessary before switching to the DSCH is possible. The benefit of the DSCH is still that the aDCH does not need to reserve low spreading factor nodes as high bitrates can be transmitted on the DSCH and the nominal bitrates on the DCH. During the packet active period, or whenever the DCH is up, the UE (and the corresponding network entity) changes from the cell_FACH state to the cell_DCH state, and during the “reading” time, or whenever all codes for a UE are released, the UE (and the corresponding network entity) changes back to the cell_FACH state. In a bursty traffic load, switching between the cell_FACH and cell_DCH states is very frequent, thereby causing a delay in packet data transmission. If the aDCH is not released, the UE (and the corresponding network entity) will remain in the cell_DCH state.
In the downlink packet switching, as described herein aDCH serves as a reverse link transport channel for the DSCH as shown in FIG. 2. Diagrammatically, the connection procedure can be represented by FIG. 3.
In order to improve the bursty traffic situation, an earlier solution, which is referred to as the pointer method, is used. In the pointer method, there is a downlink pointer channel of the lowest spreading factor 512, which acts as an aDCH for the DSCH. The pointer channel will point to another channel from a pool of shared control channels of SF 256 before the DSCH transmission begins. This shared control channel is capable of carrying all the signaling necessary for the UE to receive and decode the DSCH during an allocation period. Because the control channels are shared and the aDCH uses the minimum possible spreading factor of 512, the solution is more efficient in code-tree utilization as the traditional aDCH-DSCH case. The pointer method is feasible but still has the delays including setting up the aDCH. Now, as the aDCH uses the highest SF, it need not be released and reallocated frequently but can be kept allocated for long periods of time. One disadvantage is that in this solution the UE has to be capable of receiving, reading and decoding a large number of code channels at the same time. Thus, efficient sleep modes are needed at every possible frame instant to save power consumption of the UE.
It is advantageous and desirable to provide a method and device for fast downlink packet switching even without the strict requirement of setting up the aDCHs.